Air circulation and exhaust control system for commercial ovens

ABSTRACT

An oven (20) includes an oven enclosure (24) having a conveyor (40) mounted therein for transporting food products through the oven. A plurality of ribbon burners (100) are mounted within the oven interior (32) adjacent the path of the conveyor to heat the interior of the oven. Heated gases generated by operation of the ribbon burners are received through inlet plenums (140) under operation of a circulation fan (146) and are discharged through discharge tubes (154) extending adjacent portions of the conveyor (40) remote from the locations of the ribbon burners (100). Exhaust gases are removed from the oven interior through discharge ports (180) by exhaust blower assemblies (184) which are operated to remove exhaust gases from the oven interior at a rate proportional to the flow rate of combustion air and gaseous fuel into the ribbon burners.

This is a division of application Ser. No. 06/936,160 filed Dec. 1, 1986now U.S. Pat. No. 4,726,766.

TECHNICAL FIELD

This invention relates generally to commercial ovens of the type used inbaking bread and similar food products, and more particularly to an aircirculation and exhaust control system for commercial ovens.

BACKGROUND AND SUMMARY OF THE INVENTION

Since prehistoric times, ovens have been used in baking bread andsimilar dough products and in various other types of food preparationactivities. A wide variety of oven types have been successfullyemployed, including open hearth types, deck types, tray types, racktypes, and more recently conveyorized ovens. Conveyorized ovens areparticularly adapted to modern automated bakeries wherein conveyorsystems transport dough receiving pans through the various components ofthe bakery to produce food products on a continuous basis.

The present invention relates to an air circulation and exhaust controlsystem that is particularly adapted for use in conjunction withconveyorized ovens. In accordance with the broader aspects of theinvention, a conveyor transports dough receiving trays through anenclosure defining the oven interior. A plurality of ribbon burnerslocated at predetermined points adjacent the path of the conveyor burn amixture of natural gas and air to maintain a predetermined elevatedtemperature within the oven interior. The heated air tends to risewithin the oven interior.

An inlet plenum has inlet apertures situated above the ribbon burners. Acirculation fan draws the heated air through the inlet apertures and theinlet plenum and discharges the air through outlet passages to outletplenums. The outlet plenums direct the air to a plurality of dischargetubes situated at predetermined points adjacent the conveyor. Thus, theheated air generated by the ribbon burners is recirculated within theoven interior more effectively to heat the dough contained within thedough receiving pans and to assure that neither "hot spots" nor "coldspots" exist within the oven interior.

The enclosure surrounding the conveyor includes a top panel enclosingthe top of the oven interior. An exhaust blower assembly controls theflow of exhaust air out of the oven interior through discharge ports inthe top panel. The exhaust blower assembly speed is regulated inaccordance with the flow rate of air and gas into the ribbon burners. Inthis manner, the escape of heated gases from the oven interior isregulated to limit heat loss while simultaneously maintaining thedesired atmosphere within the oven interior.

DESCRIPTION OF DRAWINGS

A more complete understanding of the invention may be had by referenceto the following Detailed Description when taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a perspective view of an oven having an air circulation andexhaust control system incorporating the present invention in whichcertain component parts have been omitted and certain other componentparts have been broken away more clearly to illustrate certain featuresof the invention;

FIG. 2 is a top view of the oven of FIG. 1 in which the top panel of theoven enclosure has been omitted for clarity;

FIG. 3 is an exploded perspective view of one of the conveyor supportstands of the oven of FIG. 1;

FIG. 4 is a partial side view of the oven of FIG. 1 illustrating theconstruction and operation of the ribbon burners;

FIG. 5 is a further illustration of the construction and operation ofthe ribbon burners of the oven of FIG. 1;

FIG. 6 is a transverse sectional view of the oven of FIG. 1 in whichcertain component parts have been omitted for clarity;

FIG. 7 is a diagramatic illustration of the construction and operationof the oven of FIG. 1;

FIG. 8 is a top view of the oven of FIG. 1;

FIG. 9 is a side view of the oven of FIG. 1 further illustrating certaincomponents of the air circulation and exhaust control system;

FIG. 9A is a schematic view of the control circuitry of the aircirculation and exhaust control system;

FIG. 10 is a diagramatic longitudinal sectional view of the oven of FIG.1 illustrating the flow of exhaust gases therefrom;

FIG. 11 is a chart schematically illustrating the operation of theexhaust fans of the oven of FIG. 1; and

FIG. 12 is a view similar to FIG. 2 illustrating a second embodiment ofthe invention.

DETAILED DESCRIPTION

Referring now to the drawings, and particularly to FIGS. 1 and 2thereof, there is shown an oven 20 incorporating an air circulation andexhaust control system 22 according to the present invention. The oven20 includes an enclosure 24 comprising a bottom panel or floor 26, sidepanels or walls 28 and a ceiling or top panel 30. The walls 28 and thetop panel 30 preferably are formed of opposed sheet metal panels havinga layer of thermally insulating material sandwiched therebetween. Thefloor 26 may be formed from similar thermally insulated panels, or fromconcrete or the like. The floor 26, the walls 28, and the top panel 30define an oven interior 32.

A conveyor 40 is located in the oven interior 32. The conveyor 40 ispreferably constructed as illustrated and described in co-pending patentapplication Ser. No. 880,642, filed June 26, 1986, the disclosure ofwhich is incorporated herein by reference. The conveyor 40 preferablyhas a generally oval shape with relatively elongated sides andrelatively curved ends. While the particular conveyor 40 in the oven 20is a double spiral conveyor, it will be understood that other conveyorconfigurations can be used in the practice of the invention.

The conveyor 40 comprises an interior, ascending spiral 42 and anexterior, descending spiral 44. An inlet tier 46 comprising thelowermost tier of one side of the conveyor 40 extends through an inletdoor 48 of the enclosure 24. A first transfer portion 50 connects theinlet tier 46 with the interior, ascending spiral 42.

The interior, ascending spiral 42 comprises a plurality of conveyortiers including an uppermost tier 52. A second transfer portion 54connects the uppermost tier 52 of the ascending spiral 42 with an uppertier 56 of the exterior, descending spiral 44 of the conveyor 40. Thedescending spiral 44 comprises a plurality of conveyor tiers, includingan outlet tier 58 that forms the lowermost tier of the descending spiral44. The outlet tier 58 is on the opposite side of the conveyor 40 fromthe inlet tier 46, and extends outside the enclosure 24 through a door60.

As will be understood by those skilled in the art, one of the advantagesderiving from the use of a double spiral conveyor configuration is thatthe inlet tier 46 and the outlet tier 58 are situated in the same plane.This greatly facilitates both the construction and the operation of theoven 20. However, the order of ascension and decension, the location ofthe transfer portions, and the location of the inlet and outlet tiersmay be varied in accordance with the requirements of particularapplications of the invention.

In the operation of the conveyor 40 of the oven 20, dough receivingtrays T are sequentially received on the inlet tier 46. In manyinstances, the trays T will be received in the oven 20 directly from aproofer wherein dough contained in the trays T has been caused to rise.The dough receiving trays T travel along the inlet tier 46 and acrossthe first transfer portion 50 to the interior, ascending spiral 42 ofthe conveyor 40. The dough receiving trays T are transported upwardlyalong each of the tiers of the ascending spiral 42 and ultimately arriveat the uppermost tier 52.

From the uppermost tier 52, the dough receiving trays T move across thesecond transfer portion 54 to the exterior, descending spiral 44 of theconveyor 40. The food receiving trays T are thereafter transporteddownwardly, traveling along each of the tiers of the descending spiral44 to arrive at the outlet tier 58 whereupon the food receiving trays Tare transported out of the enclosure 24 of the oven 20.

Referring now to FIG. 3, the conveyor 40 of the oven 20 includes acontinuous track 70. The operating components of the conveyor 40including the track 70 are supported by a plurality of support stands 72which are preferably formed from a corrosion-resistant material such asstainless steel or "CORTEN" steel. Each support stand 72 includes abottom plate 74 secured to the floor 26 of the enclosure 24 by suitablefasteners extending through fastener receiving apertures 76. A pair ofside plates 78 are secured to and extend upwardly from the bottom plate74, and a top plate 80 connects the upper ends of the side plates 78.

A plurality of brackets 82 are secured to and extend inwardly from theside plates 78 of the support stand 72. The brackets 82 are arranged inopposed sets. A plate 84 interconnects each set of brackets 82 by meansof suitable fasteners 86 extending through fastener receiving holes 88and 90 formed in the brackets 82 and the plate 84, respectively.

A plurality of track supporting saddles 92 are each ridgedly secured toa portion of the track extending adjacent one of the support stands 72by welding or by suitable fasteners. Each saddle 92 includes a platereceiving aperture 94 which receives one of the plates 84interconnecting an opposed set of brackets 82 of the support stand 72.The aperture 94 of each saddle 92 is designed loosely to receive itscorresponding plate 84 in the horizontal direction, with the tolerancein the vertical direction being a slide fit. In this manner, theconveyor 40 of the oven 20 is securely supported within the oveninterior 32 while simultaneously accommodating thermal expansion andcontraction of the component parts of the conveyor 40 and the supportstands 72.

Referring now to FIGS. 1, 2, 4, and 5, the oven 20 further includes aplurality of ribbon burners 100. As is best shown in FIG. 5, each ribbonburner 100 comprises a V-shaped length of metal tubing 102 having a slot104 formed therein from which a mixture of natural gas and air isdischarged for combustion. The V-shaped length of metal tubing 102 issupported by a three-point suspension system. A pair of collars 106supported from frame members 108 loosely receive the tubing 102 tofacilitate thermal expansion thereof. A pin and slot connection 110supports the apex of the V-shaped length of metal tubing 102 from aframe member 112, again accommodating thermal expansion. An inletmanifold 114 extends to an elbow 116. A length of flexible tubing 118connects the elbow 116 to the V-shaped length of metal tubing 102.

As is best shown in FIG. 4, a supply of combustion air is connected to amanifold 120 through a conduit 122 having an air flow indicator 123mounted thereon. A supply of gaseous fuel, for example natural gas,butane, propane, etc., is connected to a manifold 124 through a conduit126. The manifolds 120 and 124 are in turn connected to mixing valves128 that interconnect the conduits 122 and 126 and the inlet manifolds114.

The gaseous fuel is provided at a pressure of one atmosphere absolute.Air passing through the mixing valves 128 and the inlet manifolds 114causes a pressure differential between the flowing air and the gaseousfuel. This pressure differential draws gaseous fuel into the mixingvalves 128 and the inlet manifolds 114 thereby receive a combustiblemixture from the mixing valves 128 comprising about ten parts combustionair and one part gaseous fuel as determined by the setting of the valves128. The air flow indicator 123 senses the air flow volume and providesthe information to a computer 129.

As is best shown in FIGS. 2, 4 and 5, the combustible mixture flowsthrough the inlet manifolds 114, the flexible tubing 118 and theV-shaped length of metal tubing 102 and is discharged through the slot104. Each ribbon burner 100 is provided with an ignitor 130 situated atone end of the slot 104. The ignitors 130 comprise spark plugs that areactuated by transformers situated within a housing 132. Upon actuation,the ignitors 130 ignite the combustible mixture discharged through theslots 104 of the V-shaped lengths of metal tubing 102.

Referring specifically to FIG. 2, safe operation of the oven 20 requiresconstant surveillance to insure that each of the ribbon burners 100 isignited. This function is provided by flame sensors 134 mounted in thewalls 28 of the enclosure 24. The flame sensors 134 comprise opticaldevices that are focused on the ends of the slots 104 of the lengths ofV-shaped metal tubing 102 remote from the ignitors 130. The flamesensors 134 normally provide an electrical signal indicative of thepresence of flame at the ends of the slots 104 remote from the inletends thereof.

Referring now to FIGS. 1 and 2, the air circulation and exhaust controlsystem 22 of the oven 20 includes a pair of inlet plenums 140 eachhaving an outer inlet aperture 142 and an inner inlet aperture 144. Theinlet plenums 140 extend to a circulation fan 146 located inside theoven enclosure 24 which is driven by a motor 148 mounted on top of thetop panel 30 of the enclosure 24 and, therefore, outside the oveninterior 32. The circulation fan 146 draws heated gases into the inletplenums 140 through the inlet apertures 142 and 144 thereof anddischarges the heated gases through outlet passages 150. The outletpassages 150 extend to outlet plenums 152 which in turn extend todischarge tubes 154. Each discharge tube 154 includes a relatively largediameter inner section 156 and a relatively small diameter outer section158. Both the inner section 156 and the outer section 158 of thedischarge tube 154 have a plurality of outlet apertures 160 formedtherein.

The inner section 156 of each discharge tube is rotatably positionablerelative to its respective outlet plenum 152 so that the direction ofdischarge of heated gases through the outlet apertures 160 in each innersection 156 is adjustable in accordance with the operating parameters ofthe particular oven installation. Likewise, the outer section 158 ofeach discharge tube 154 is rotatably positionable relative to the innersection 156 thereof, again to direct the discharge of heated gasestherefrom as may be required by particular applications of theinvention.

Referring to FIGS. 1 and 2, the conveyor 40 has a generally oval shapecomprising relatively long, straight side portions and relatively curvedend portions. The ribbon burners 100 are adjacent the relativelystraight side portions of the tiers of the conveyor 40. Operating theribbon burners 100 creates heated gases comprising both the products ofcombustion and air which has been heated by contact with and mixtureinto the products of combustion. These heated gases tend to flowupwardly within the oven interior 32 and therefore flow into the inletapertures 142 and 144 of the inlet plenums 140 of the air circulationand exhaust control system 22.

The outlet plenums 152 of the air circulation and exhaust control system22 are semi-cylindrical. The discharge tubes 154 extend radiallyoutwardly from the outlet plenums 152 to points situated adjacent therelatively curved ends of the tiers of the conveyor 40. Thus, the aircirculation and exhaust control system 22 functions in one aspect towithdraw heated gases from adjacent the relatively straight sideportions of the conveyor 40 and to recirculate and discharge the heatedgases in regions adjacent to the curved end portions of the conveyor 40.

FIG. 7 diagramatically illustrates the path of the dough receiving traysT through the oven 20 and the relationship of the moving dough receivingtrays T to the ribbon burners 100 and the discharge tubes 154. A ribbonburner 100 underlies the inlet tier 46 of the conveyor 40. The firsttransfer portion 50 of the conveyor 40 directs the trays T to theinterior, ascending spiral 42. Ribbon burners 100 underlie lowerportions of the ascending spiral 42. Both discharge tubes 154 and ribbonburners 100 underlie the upper portions of the ascending spiral 42.Thus, heating of the food products carried by the dough receiving traysfrom the underside thereof is concentrated when the trays T aretraveling upwardly on the ascending spiral 42 and particularly when thetrays are traveling on the upper portions of the ascending spiral 42.

Having reached the top of the interior, ascending spiral 42, the trays Ttravel across the second transfer portion 54 to the exterior, descendingspiral 44 of the conveyor 40. As the dough receiving trays T and thefood products contained therein travel downwardly on the descendingspiral 44 of the conveyor 40, the trays pass under the discharge tubes154 and over the ribbon burners 100. The positioning of the dischargetubes 154 above the trays T and the products carried therein on thedescending spiral 44 of the conveyor 40 is an important feature of theinvention.

Initially, the dough is wet and tender. It is important to heat thedough and cause it to lose water before putting a crust on the dough.The discharge tubes 154 are therefore placed beneath the interior,ascending spiral 42 of the conveyor 40. The concentration of dischargetubes 154 and ribbon burners 100 is greater around the inside loop thanaround the outside loop to assist in driving water out of the dough andtoughening the dough. On the exterior, descending spiral 44, thedischarge tubes 154 are above the dough receiving trays T to assist inbuilding a crust and browning the dough products. It will be understood,however, that the direction of discharge of heated gases from the tubes154 depends upon the nature of the food products being prepared in theoven.

Referring now to FIGS. 1 and 6, the flow of the hot gases within theoven interior 32 is shown. The heated gases caused by operation of theribbon burners 100 flow into the inlet apertures 142 and 144 and intothe inlet plenums 140 as shown by the arrows 170. The gases flow out ofthe discharge tubes 154 as shown by the arrows 172.

In FIG. 1 the flow from the orifices 160 is shown as directedhorizontally downwardly onto the exterior, descending spiral 44 andvertically upwardly toward the bottom of the interior, ascending spiral42 of the conveyor 40. In actual practice, the orifices 160 of therelatively large diameter inner sections 156 and the relatively smalldiameter outer sections 158 of each of the discharge tubes 154 arepositioned to eliminate local "hot spots" and "cold spots" found in eachindividual oven made according to the present invention. For instance,the heated gases from the outlet apertures 160 in certain of thedischarge tubes 154 may be directed toward the back of the doughreceiving trays to adjust for the speed of the conveyor and eliminatecold spots.

The direction of the flow of gases through the outlet apertures 160 ofthe discharges tubes 154 is critical to successful operation of thepresent invention. As those of skill in the art know, different conveyorspeeds, different gas and air flow rates, different oven temperaturesand different types of dough and product, for instance, fermentationdough versus continuous dough, dough sprayed with water, seeds on top ofthe dough, etc., all affect the baking of the product in any given oven.Therefore, the rotational positioning of the outlet apertures 160 asindicated by the arrows 172 must be individually adjusted by a skilledoven operator for each of the discharge tubes 154 in each oven and foreach product produced in the oven.

Referring now to FIGS. 2 and 8, a pair of discharge ports 180 are formedin the top panel 30 of the enclosure 24 of the oven 20. A pair ofconduits 182 each extend from one of the discharge ports 180 to anexhaust blower assembly 184. The exhaust blower assemblies 184 withdrawheated gases from the oven interior 32 and discharge the heated gasesinto the atmosphere through stacks 186.

As is best shown in FIG. 9, each blower assembly 184 includes a blowershaft 190 having a sprocket 192 secured thereto. A timing belt 194drivingly connects the sprocket 192 with a sprocket 196 which in turndrives a speed switch 198 and a tachometer 200. The tachometer 200provides the computer 129 with an electrical output analogous to therotational speed of a blower shaft 190, and the speed switch 198functions as a governor to prevent operation of the ribbon burners 100until the blower assembly 184 is operating at a sufficiently highrotational speed.

In accordance with the present invention, the rate of flow of combustionair into the manifold 120 is continuously monitored by the air flowindicator 123 and the flow rate is provided to the computer 129. Theflow rate of the gaseous fuel is proportional to the flow rate of thecombustion air and is determined by the setting of the mixing valves128. The computer 129 compares the combined flow rate with the exhaustflow rate as indicated by the tachometer 200. The computer 129 thenadjusts the speed of each exhaust blower assembly 184 so that theoperational speed of each exhaust blower assembly 184 is maintained at alevel that results in an exhaust flow rate that is roughly 7% above thecombined flow rate of the combustion air and the gaseous fuel into theoven interior 32. Thus, a slight negative pressure is maintained at thedoors 48 and 60 through which the trays T enter and leave the oveninterior 32.

As is shown in FIG. 11, the exhaust blower assembly 184 of the aircirculation and exhaust control system 22 has several operationalranges. These include a standby range 202, a baking range 204, a reserverange 206 and a purge range 208. The standby range 202 is used when theoven is in a heated condition but no food products are being transportedtherethrough by the conveyor 40. The baking range 204 is the normaloperational range of the exhaust blower assembly 184 when the oven 20processes food products that are transported therethrough on the trays Tby the conveyor 40. The reserve range 206 is normally not used, but isprovided for those circumstances in which the operational conditions ofthe oven dictate a greater exhaust flow rate than is normally required.The purge range 208 is used to remove all heated gases from the interiorof the oven, for example, during emergency conditions, or when it isnecessary to prepare the oven for maintenance, etc.

The relationship between the recirculation components and the exhaustcontrol components of the air circulation and exhaust control system 22of the oven 20 is further illustrated by reference to FIG. 10. Under theminimum operating conditions of the oven 20, the combined flow rate ofcombustion air and gaseous fuel into the ribbon burners 100 is 360 cubicfeet per minute. The flow rate of heated gases through the inlet plenums140, the outlet passages 150, the outlet plenums 152, and the dischargetubes 154 under the action of the circulation fan 146 as indicated bythe arrows 210 is 2400 cubic feet per minute. The flow rate of gases outof the oven through the discharge ports 180 as indicated by the arrows212 is 390 cubic feet per minute. The rate of flow of gases returned tothe burner area as indicated by the arrows 214 is 2010 cubic feet perminute. The flow rate of air through the doors 48 and 60, which alsoflows to the burner area is 30 cubic feet per minute.

When the oven is operated under standby conditions, that is, when theoven is heated but no food products are being transported therethroughon the trays T under the action of conveyor 40, the combined flow rateof combustion air and gaseous fuel into the ribbon burners 100 is 600cubic feet per minute. The recirculation rate as indicated by the arrows210 is 2400 cubic feet per minute, the exhaust flow rate as indicated bythe arrows 212 is 640 cubic feet per minute and the rate of return ofgases to the burner area as indicated by the arrows 214 is 1760 cubicfeet per minute. The rate of flow of air through the doors 48 and 60 is40 cubic feet per minute.

When the oven 20 is operated in the baking condition, the combined flowrate of combustion air and gaseous fuel into the ribbon burners 100 is920 cubic feet per minute, the recirculation rate as indicated by thearrows 210 is 2400 cubic feet per minute, the exhaust flow rate asindicated by the arrows 212 is 990 cubic feet per minute and the rate offlow of gases back to the burner area as indicated by the arrows 214 is1410 cubic feet per minute. The rate of flow of air through the doors 48and 60 is 70 cubic feet per minute.

When the oven 20 is operated under maximum heating conditions, thecombined flow rate of combustion air and gaseous fuel into the ribbonburners 100 is 1065 cubic feet per minute, the recirculation rate asindicated by the arrows 210 is 2400 cubic feet per minute, and theexhaust flow rate as indicated by the arrows 212 is 1140 cubic feet perminute. Under such conditions, gases are returned to the burner area asindicated by the arrows 214 at the rate of 1260 cubic feet per minute,and the rate of air flow through the doors 48 and 60 is 75 cubic feetper minute.

When the oven 20 is operated under purging conditions, that is, when itis desired to completely exhaust heated gases from the oven interior 32,the ribbon burners 100 are not operated and neither combustion air norgaseous fuel is admitted thereto. The recirculation rate as indicated bythe arrows 210 is 2400 cubic feet per minute, and the exhaust flow rateas indicated by the arrows 212 is 3800 cubic feet per minute. Under suchconditions, heated gases are withdrawn from the burner area in thedirection opposite that indicated by the arrows 214 at a rate of 1400cubic feet per minute. The rate of air flow into the oven interior is3800 cubic feet per minute.

Referring now to FIG. 12, an oven 220 comprising a second embodiment ofthe invention is illustrated therein. The oven 220 incorporates numerouscomponent parts which are substantially identical in function andconstruction to component parts of the oven 20 as illustrated in FIGS.1-11 and described above. Such identical component parts are designatedin FIG. 12 with the same reference numerals utilized to designate thecomponent parts of the oven 20, but are differentiated therefrom bymeans of a prime (') designation. The oven 220 differs from the oven 20comprising the first embodiment of the invention primarily in the factthat it incorporates dual inlet plenums 222 and 224 and dual circulationfans 226 and 228 leading to the outlet passages 150', the outlet plenums152' and the discharge tubes 154'. This allows control of thecirculation rate of heated gases to the opposite ends of the oveninterior 32' by regulating the operational speeds of the circulationfans 226 and 228 rather than the use of dampers as might otherwise berequired in those embodiments of the invention employing a singlecirculation fan.

Although preferred embodiments of the invention have been illustrated inthe accompanying drawings and described in the foregoing DetailedDescription, it will be understood that the invention is capable ofnumerous rearrangements, modifications and substitutions of parts andelements without departing from the spirit of the invention.

We claim:
 1. A heated air circulation and exhaust control system for anoven comprising:means forming an oven enclosure defining an oveninterior; conveyor means for transporting food products through the oveninterior along a predetermined spiral path having relatively elongatedside portions and curved end portions; inlet apertures situated adjacentthe side portions within the oven interior for receiving heated gases;discharge tubes having outlet apertures situated within the oveninterior adjacent to the conveyor path at the curved end portions; andcirculation means coupling said inlet apertures to said discharge tubesfor causing the flow of heated gases from the inlet aperture to theoutlet apertures of the discharge tubes to heat the food products on theconveyor in the curved end portions.
 2. The air circulation and exhaustcontrol system of claim 1 further comprising:an outlet plenum connectedto the circulation means; and a plurality of discharge tubes extendingradially outwardly from the outlet plenum to points adjacent torelatively curved ends of the conveyor.
 3. The air circulation andexhaust control system of claim 2 wherein the conveyor means followssaid spiral path in a plurality of tiers and wherein at least some ofthe discharge tubes are located between predetermined tiers of theconveyor means.
 4. The air circulation and exhaust control system ofclaim 3 wherein each discharge tube has an inner portion and an outerportion each selectively positionable to control the direction ofdischarge of heated gases therefrom.
 5. The air circulation and exhaustcontrol system as in claim 4 further including:means for positioningsaid inner portion of said discharge tube to heat said conveyor on saidinner ascending spiral from the bottom to remove water from the foodproducts traveling thereon; and means for positioning the outer portionof the discharge tube to heat said conveyor on said outer descendingspiral from the top to brown the food products traveling thereon.
 6. Anair circulation and exhaust control system as in claim 8 furthercomprising:means for generating a signal indicative of the combined rateof flow of combustion air and gaseous fuel into the burner means;discharge port means for directing exhaust gases out of the oveninterior; exhaust blower means for controlling the rate of flow ofexhaust gases out of the interior through the discharge port means; andmeans for operating the exhaust blower means to withdraw exhaust gasesfrom the oven interior through the discharge port at a rate proportionalto the rate of flow of combustion air and gaseous fuel into the burnermeans.
 7. The air circulation and exhaust control system as in claim 1further comprising:means for generating a signal indicative of thecombined rate of flow of combustion air and gaseous fuel into the burnermeans; discharge port means for directing exhaust gases out of the oveninterior; exhaust blower means for controlling the rate of flow ofexhaust gases out of the interior through the discharge port means; andmeans for operating the exhaust blower means to withdraw exhaust gasesfrom the oven interior through the discharge port at a rate proportionalto the rate of flow of combustion air and gaseous fuel into the burnermeans.
 8. An air circulation and exhaust control system for an ovencomprising:means forming an oven enclosure defining an oven interior;conveyor means including relatively elongated side portions and circularend portions for transporting food products through the oven interioralong a predetermined path; burner means situated within the ovenadjacent at least one of the elongated sides for receiving combustionair and gaseous fuel and for burning the combustion air and the gaseousfuel to heat the oven interior; an inlet aperture situated within theoven interior and superimposed above the burner means for receivingheated gases generated by operation of the burner means; at least onedischarge tube having an outlet aperture located adjacent at least oneof the circular end portions of the conveyor path situated remote fromthe side portions thereof for effective heating of the food products onthe conveyor paths; and circulation means coupling said inlet apertureand said discharge tube for causing a flow of heated gases from theinlet aperture near the elongated side to the discharge tube near thecircular end portion.
 9. The air circulation and exhaust control systemof claim 8 wherein said burner means comprises a ribbon burner.
 10. Theair circulation and exhaust control system of claim 9 wherein the ribbonburner is V-shaped.
 11. The air circulation and exhaust control systemof claim 14 wherein each of the V-shaped ribbon burners is mountedbeneath at least one of the elongated side portions of the conveyormeans and is supported by a three point suspension system including apair of collars and a connection at the apex of the V-shaped ribbonburner such that each of the connections is made to accommodate thermalexpansion.
 12. The air circulation and exhaust control system of claim11 further comprising:an ignitor at one end of each V-shaped ribbonburner; and a flame sensor at the opposite end of each V-shaped ribbonburner.
 13. The air circulation and exhaust control system of claim 12wherein the conveyor means comprises a plurality of tiers and whereineach of the burner means is located beneath a side portion of one tierof the conveyor means.
 14. The air circulation and exhaust controlsystem of claim 8 wherein the conveyor means path comprises an interior,ascending spiral and an exterior, descending spiral.
 15. The aircirculation and exhaust control system of claim 14 wherein the dischargetube has an inner portion associated with the interior ascending spiraland an outer portion associated with the outer descending spiral, eachportion being selectively positionable to control the direction ofheated gas discharge relative to products traveling both on theinterior, ascending spiral and on the exterior, descending spiral. 16.The air circulation and exhaust control system of claim 8 furthercomprising:dual inlet plenums extending to dual inlet apertures situatedabove the burner means for receiving heated gases generated by operationof the burner means; and dual circulation fan means for drawing heatedgases into the dual inlet plenums through the inlet apertures thereofand for directing the heated gases through dual conduit means to thedischarge tube and outwardly through the discharge aperture thereof toeffect heating of the food products.
 17. The air circulation and exhaustcontrol system as in claim 15 further including:means for positioningsaid inner portion of said discharge tube to heat said conveyor on saidinner ascending spiral from the bottom to remove water from the foodproducts traveling thereon; and means for positioning the outer portionof the discharge tube to heat said conveyor on said outer descendingspiral from the top to brown the food products traveling thereon.